1. Field of the Invention
The present invention relates to a hydraulic shock absorber for use in a suspension system of a vehicle, for example, an automobile.
2. Description of the Prior Art
A typical conventional hydraulic shock absorber has heretofore been arranged such that the flow of a hydraulic fluid that is induced in a passage providing communication between two chambers by the sliding motion of a piston in a cylinder is controlled by an orifice passage and a disk valve mechanism, thereby generating a damping force. When the piston speed is low, the shock absorber shows damping force characteristics (orifice characteristics), whereby the damping force changes along a quadratic curve in accordance with the piston speed by virtue of the throttling action of the orifice passage, and when the piston speed exceeds a predetermined level, the shock absorber shows damping force characteristics (valve characteristics) whereby the damping force changes linearly in accordance with the piston speed.
Incidentally, in order to make a vehicle more comfortable to ride in and to improve the steering stability of the vehicle, orifice and valve characteristics such as those described below are desired: When the vehicle is running in a normal state where the unsprung part of the suspension system is subject to fine vibration (i.e., high-frequency vibration), orifice and valve characteristics that provide a relatively small damping force are desired, whereas, when the vehicle is making a turn or being braked, that is, when the unsprung part of the suspension system is subject to relatively slow and large vibration (i.e., low-frequency vibration), orifice and valve characteristics that provide a relatively large damping force are desired.
Under these circumstances, for example, Japanese Patent Post-Exam. Publication No. 42-17787 (1967) discloses a hydraulic shock absorber wherein the valve characteristics change in accordance with the frequency of vibration under the spring of the suspension system. In this hydraulic shock absorber, when the frequency of vibration is low, the injection valve opening pressure of a disk valve is raised to generate a large damping force, whereas, when the frequency of vibration is high, the injection valve opening pressure of the disk valve is lowered to reduce the damping force, thereby making the vehicle even more comfortable to ride in and improving the steering stability.
Japanese Utility Model Public Disclosure (KOKAI) No. 62-25346 (1987) discloses a hydraulic shock absorber wherein the orifice characteristics at the extension side change in accordance with the frequency of vibration under the spring of the suspension system. In this hydraulic shock absorber, when the frequency of vibration is low, the passage area of the orifice is reduced to generate a large damping force, whereas, when the frequency of vibration is high, the orifice passage area is increased to reduce the damping force, thereby making the vehicle even more comfortable to ride in and improving the steering stability.